Mehod for gradual sealing of a gas

ABSTRACT

A method is proposed for compressing a gas in stages in a compressor arrangement (100, 200, 300, 400) having a plurality of compression stages (I-VI) which are connected together sequentially by a main line (1) and in which the gas, guided through the main line (1), is respectively compressed from a suction-side pressure level to a pressure-side pressure level and is heated by this compression from a suction-side temperature level to a pressure-side temperature level, wherein a feedback amount of the gas, guided through the main line (1), is at least temporarily removed from the main line (1) downstream of one of the compression stages (V), is fed to an expansion process, and is fed back into the main line (1) upstream of the same compression stage (V). It is provided that the pressure-side pressure level of the compression stage (V) downstream of which the feedback amount is removed from the main line (1) is a supercritical pressure level, that the feedback amount is expanded to a subcritical pressure level, that the feedback amount is fed to the expansion process at the pressure-side temperature level of the compression stage (V) downstream of which it is removed from the main line (1), and that the feedback amount is cooled only after being expanded and before and/or after being fed back into the main line (1). The invention also relates to a compressor arrangement (100, 200, 300, 400).

The invention relates to a method for compressing a gas in stages in a compressor arrangement having a plurality of compression stages which are connected together sequentially by a main line, and to a corresponding compressor arrangement according to the preamble of the independent claims.

PRIOR ART

Methods and devices for steam cracking hydrocarbons are known and are described for example in the article “Ethylene” in Ullmann's Encyclopaedia of Industrial Chemistry, online since 15 Apr. 2009, DOI 10.1002/14356007.a10_045.pub3.

In steam cracking, gas mixtures are obtained which, after separating water and oily constituents, if present (so-called pyrolysis oil), still substantially contain hydrogen, methane and hydrocarbons having two and more carbon atoms. Gas mixtures of this type can be separated in separating sequences, as are basically known to a person skilled in the art and are also described in the mentioned article. The conventional target product in steam cracking, namely ethylene, is also separated from the other components in appropriate separating sequences. In this respect, ethylene is typically drawn off from the top of a so-called C2 splitter.

Subject to the requirements of downstream processes or consumers, ethylene can be released to the plant limit under different conditions. Typically, ethylene is released under elevated pressure in the form of gas. Provided that the ethylene is not already present under the desired conditions, compression is carried out in single-stage or multi-stage turbocompressors. Since ethylene is also used as a refrigerant in the mentioned separating sequences, it is also compressed for this purpose. Ethylene is typically compressed for the mentioned purposes in a common turbocompressor having a plurality of compression stages and intermediate cooling, which is thus also used as the product compressor and as the refrigerant compressor. The ethylene is removed from this turbocompressor for use as refrigerant and as product at different pressure levels corresponding to different compression stages. A corresponding turbocompressor is shown schematically in the accompanying FIG. 1 and is described in more detail below. However, in principle it is also possible to use a plurality of separate, in particular multi-stage, turbocompressors for refrigerant and product compression.

Although the present invention is mainly described with reference to ethylene and to ethylene-rich gases, it is equally suitable for the compression of gases which behave similarly thermodynamically, such as ethane and carbon dioxide. The description with regard to ethylene is therefore used only as an example.

In particular cases, the release of ethylene at a supercritical pressure level is required. For this purpose, the ethylene can either be liquefied and then conveyed at a supercritical pressure level by a pump, or it is brought to the corresponding pressure level in a multi-stage compressor of the described type. The latter case is shown schematically in the accompanying FIG. 2 and is also described in more detail below. Here, the product is compressed in stages IV to VI and the refrigerant is compressed in stages I to III. Compression without liquefaction is the more favourable alternative in terms of energy.

However, problems can arise in multi-stage compression at supercritical pressure levels in conventional multi-stage turbocompressors, as described below. The present invention is intended to overcome these problems.

DISCLOSURE OF THE INVENTION

In view of the above, the invention proposes a method for compressing a gas in stages in a compressor arrangement having a plurality of compression stages which are connected together sequentially by a main line, and a corresponding compressor arrangement having the features of the independent claims. Embodiments are the subject of the dependent claims and of the following description.

To characterise pressures and temperatures, the present application uses the terms “pressure level” and “temperature level”, which are intended to signify that corresponding pressures and temperatures in a corresponding plant do not have to be used as exact pressure and temperature values in order to realise the inventive concept. However, such pressures and temperatures are typically within particular ranges which lie, for example ±1%, 5%, 10%, 20% or even 50% around an average. In this respect, corresponding pressure levels and temperature levels can lie within disjoint ranges or within overlapping ranges. In particular, for example pressure levels include pressure losses which are unavoidable or which are to be expected. The same applies accordingly to temperature levels. Pressure levels which are stated here in bar are absolute pressures.

Advantages of the invention

Before the ethylene reaches the supercritical pressure level in a multi-stage turbocompressor, it is possible for liquefaction to occur at relatively moderate temperatures at a high, but still subcritical pressure level. The critical temperature of ethylene is approximately 8° C. This temperature is low enough for liquefaction to be ruled out by an intermediate cooling with cooling water downstream of a compression stage. Cooling water is typically at a temperature of at least 10° C. Howevor, typically provided in multi-stage turbocompressors are return lines, or so-called kickbacks, which, under partial load or other intermittently occurring operating states, expand ethylene to a lower pressure level from the pressure side of a compression stage and feed it back on the suction side to the same compression stage or to a compression stage which is arranged upstream thereof. This is also shown in FIG. 2 and is described in more detail below.

If the ethylene is cooled too much before a corresponding recirculation, an undesirable partial liquefaction can result during expansion. If the compressed ethylene is at approximately 70 bar for example after stage V of the turbocompressor according to FIG. 2, and if it is cooled to 20° C. at this pressure level and then expanded isenthalpically to approximately 40 bar by a throttle to feed it back to stage V, a direct transition into the two-phase region results in the enthalpy diagram. For details, reference is made to enthalpy diagrams for ethylene published in the relevant specialist literature. However, a corresponding partial liquefaction is a disadvantage, because the multi-stage turbocompressors which are used are not configured for conveying liquid phases.

To overcome this disadvantage, the present invention proposes a method for compressing a gas in stages in a compressor arrangement having a plurality of compression stages which are connected together sequentially by a main line. The compression stages can be configured in particular as turbocompression stages, as previously described. In particular, the compression stages can be partly or entirely driven by common mechanical devices, for example common shafts, and in this way they are coupled together mechanically.

As mentioned, the gas used in the present invention can be, for example, an ethylene-rich gas. An ethylene-rich gas of this type can also be pure or substantially pure ethylene, i.e. it can contain at least 90%, 95% or 99% ethylene. Since in the context of the present invention the ethylene-rich gas can be removed in particular from the top of a known C2 splitter (see the specialist literature mentioned at the beginning), it has in particular the usual ethylene content in this connection. For simplification purposes, a corresponding ethylene-rich gas will also be referred to in the following as “ethylene”. However, the present invention is also suitable, for example, for the compression of ethane or carbon dioxide, which have comparable thermodynamic characteristic quantities, or for corresponding ethane-rich and carbon dioxide-rich gases.

In the compression stages, the gas which is guided through the main line is respectively compressed from a suction-side pressure level to a pressure-side pressure level and is heated by this compression from a suction-side temperature level to a pressure-side temperature level.

Here, the term “suction-side pressure level” is understood as meaning the pressure level at which the gas is fed to the compression stage. This suction-side pressure level is also commonly known as “suction pressure”. The “pressure-side pressure level” is the pressure level to which the compression stage compresses the gas.

Here, the term “suction-side temperature level” is understood as meaning the temperature level at which a corresponding gas is fed to the compression stage. This temperature level is no longer actively influenced before the gas is fed into the compressor, in particular the gas is no longer actively heated or cooled from a suction-side temperature level. Accordingly, a “pressure-side temperature level” denotes the temperature level directly downstream of a corresponding compression stage, thus the pressure-side temperature level is the pressure level at which a corresponding gas leaves the compression stage. Therefore, downstream of the compression stage, the temperature level is no longer actively influenced to reach the pressure-side temperature level, in particular there is no heating or active cooling in a cooler. If an intermediate cooler is used downstream of the compression stage, the “pressure-side temperature level” is present up to the entry of the gas into the intermediate cooler.

Within the context of the present invention, according to conventional kickback lines, a feedback amount of the gas guided through the main line is at least temporarily removed from the main line downstream of one of the compression stages, is fed to an expansion process and after expansion is fed back into the main line upstream of the same compression stage.

The expression feedback “upstream of the same compression stage” can mean, as also explained below, feedback directly upstream of the compression stage downstream of which the feedback amount was removed; however, feedback can also take place upstream of one or more further compression stages which are arranged upstream of the compression stage downstream of which the feedback amount was removed from the main line.

As already stated, if appropriate kickbacks are used in the form of the described feedback amount, the previously described problems of partial liquefaction can occur when an arrangement is used which has been previously described and is shown in FIG. 2. In such an arrangement, an appropriate feedback amount is removed from the main line downstream of an aftercooler so that it is already in a cooled state at the removal point. The feedback amount is further cooled by the expansion process, which is necessary for feedback. This is particularly critical in the case of fluctuating or generally low cooling water temperatures, because (partial) liquefaction can result here. As mentioned, this can take place for example in the case of ethylene or ethylene-rich gases even at relatively moderate temperatures, namely for example if the feedback amount is at a pressure of approximately 70 bar, is cooled to 20° C. at this pressure level and is then expanded isenthalpically to approximately 40 bar by a throttle.

A corresponding liquid phase formation cannot exclusively occur when the pressure-side pressure level of the compression stage downstream of which the feedback amount is removed from the main line is above a supercritical pressure level and this feedback amount is expanded to a subcritical pressure level, but during normal operation of a corresponding plant it is a disadvantage particularly during a “transcritical” expansion of this type. Therefore, the present invention focuses on corresponding cases of transcritical compression and expansion.

The invention therefore provides that the pressure-side pressure level of the compression stage downstream of which the feedback amount is removed from the main line is a supercritical pressure level, that the feedback amount is expanded to a subcritical pressure level and that the feedback amount is fed to the expansion process at the pressure-side temperature level of the compression stage downstream of which it is removed from the main line. To obtain the advantages according to the invention, the feedback amount is cooled only after being expanded and before and/or after being fed back into the main line.

In principle, corresponding problems can also arise if the feedback amount is expanded during normal operation from a supercritical to a supercritical pressure level (i.e. for example in stage VI of the arrangement shown in FIG. 2 or of a corresponding feedback amount). For example, during the start-up or during a malfunction of a corresponding plant, the pressure level of the feedback can temporarily lie below the critical pressure level or can fall to a corresponding value. During start-up or during disruptions, considerable fluctuations in the pressure levels can potentially be recorded until a corresponding plant has (again) reached a steady state.

An advantageous embodiment of the method according to the invention therefore provides that a further feedback amount of the gas, guided through the main line, is at least temporarily removed from the main line downstream of a further compression stage, is fed to an expansion process, and is fed back into the main line upstream of the same further compression stage, that the pressure-side pressure level of the further compression stage downstream of which the further feedback amount is removed from the main line is a supercritical pressure level, that this further feedback amount is expanded to a supercritical pressure level, and that the feedback amount is cooled only after being expanded and before and/or after being fed back into the main line. In this way, liquefaction is also avoided in this case.

In particular cases, liquefaction can also occur when a pressure-side pressure level is below the supercritical pressure level, namely when the feedback amount before expansion is at a subcritical pressure level and simultaneously at a temperature level at which the two-phase region can be achieved by simple expansion. An example of this is a pressure level of approximately 48 bar and a temperature level of approximately 10° C. In the pressure enthalpy diagram, a point defined by a corresponding pressure level and temperature level is located above the two phase line.

To avoid liquefaction in this case as well, a further embodiment of the present invention provides that an additional feedback amount of the gas, guided through the main line, is at least temporarily removed from the main line downstream of an additional compression stage, is fed to an expansion process, and is fed back into the main line upstream of the same additional compression stage, that the pressure-side pressure level of the additional compression stage downstream of which the further feedback amount is removed from the main line is a subcritical pressure level, that this additional feedback amount is expanded to a subcritical pressure level, and that the feedback amount is cooled only after being expanded and before and/or after being fed back into the main line.

The present invention solves the problem of liquefaction in the mentioned cases in that the feedback amount (the following explanations also relate to a plurality of feedback amounts) is fed to the expansion process at the pressure-side temperature level of the compression stage downstream of which it is removed from the main line. In other words, within the context of the present invention, a corresponding feedback amount is not cooled downstream of the relevant compression stage at which the feedback amount is formed, before the expansion thereof. This is an essential difference over the prior art. In the case of an isenthalpic expansion starting from a corresponding pressure level, but from a higher temperature than previously mentioned (because aftercooling has still not taken place), the two-phase region of a corresponding enthalpy diagram is not attained and therefore the feedback amount remains fully in the gaseous state. As already stated, the expansion preferably takes place isenthalpically, i.e. a throttle valve is preferably used for the expansion.

As already mentioned, in the context of the present invention, cooling does not take place before the expansion of the feedback amount after it has been removed from the main line. However, within the context of the present invention, the feedback amount is cooled following expansion and before and/or after being fed back into the main line, for which purpose a separate heat exchanger can be provided in a return line used for returning the feedback amount and/or in the main line downstream of the infeed point of the feedback amount. A separate heat exchanger of this type can be advantageous, because in this way the cooling of a corresponding feedback amount can be adapted individually to the respectively required conditions. However, it is also possible to carry out a cooling process in the main line without a separate heat exchanger. In this case, the feedback amount is fed into the main line without cooling or after (partial) cooling in a separate heat exchanger in the return line and is cooled there by the heat exchanger, which is also used for cooling the remaining gas which has not been fed back and is present in the main line. In this way, a corresponding plant can be set up in a relatively simple and cost-effective manner.

As already mentioned, within the context of the present invention the feedback amount does not necessarily have to be fed back into the main line directly upstream of the compression stage downstream of which it was removed from the main line. Instead, the feedback amount can be advantageously fed back into the main line upstream of one or more compression stages which are arranged upstream of the compression stage downstream of which the feedback amount is removed from the main line. In this way, the suction-side pressure levels of a plurality of upstream compression stages can be influenced in a particularly advantageous manner using a feedback amount.

Within the context of the present invention, the feedback amount is advantageously controlled based on an attainable or attained suction-side or pressure-side pressure level of one of the compression stages. In particular, in a corresponding method, the product pressure can be fixed by a controlled valve which is arranged downstream of the last compression stage. This valve fixes the product pressure, i.e. the pressure-side pressure level of the last compression stage. A pressure level of this type can be for example approximately 125.6 bar. In this case, the pressure-side temperature level, downstream of an aftercooling downstream of the last compression stage, can be for example approximately 40° C.

The suction-side pressure level of an upstream compression stage which is charged with ethylene from a high pressure ethylene refrigerant circuit and which compresses the gas in the main line to a pressure level of, for example, approximately 22.5 bar can be adjusted by the rotational speed of this compression stage. The suction-side pressure level of the compression stages arranged upstream thereof is also fixed thereby in a corresponding multi-stage turbocompressor. If a corresponding suction-side pressure level is not attained, for example during partial load, a control can be carried out by opening appropriate kickbacks, i.e. by providing or increasing an appropriate feedback amount.

As also explained in the following with reference to the accompanying figures, in particular operating states, for example during partial load or at varying cooling water flow temperatures, the entry conditions of the individual compression stages can vary to different extents. In addition, the thermodynamic characteristics of the fluid under elevated pressure can have a disproportionate effect. Consequently, downstream compression stages in a corresponding multi-stage turbocompressor increasingly generate too much pressure, possibly during partial load or if the cooling water is too cold. To solve this problem as well, a control can be carried out by adjusting appropriate feedback amounts.

The present invention is particularly advantageous in cases of fluctuating cooling water temperatures, because within the context of the present invention the temperature of the feedback amount can no longer fall below the cooling water temperature, because this is firstly expanded and is only then cooled. However, in the case of cooling (not according to the invention) of the feedback amount before expansion, the temperature level will fall below the cooling water temperature, because further cooling takes place starting from the temperature reached by cooling. This is a particular disadvantage in cases in which, as explained, for example during partial load, the suction-side pressure level of a compression stage is set by the opening the provision or increase of the feedback amount. The pressure-side temperature level of such a compression stage becomes increasingly cold by increasing the correspondingly cold feedback amount. This produces disadvantages in terms of control in this compression stage and in the downstream compression stages.

As an alternative or in addition to the described measures, to overcome this problem, it can be provided to cool the gas between the compression stages using cooling water which is maintained within a predetermined temperature range. The rotational speed of compression stages can also be controlled separately in the high pressure range in this case.

As already mentioned, within the context of the present invention it is possible for a plurality of the compression stages to be driven by one or more common shafts, to which the respective compression stages are mechanically coupled. Appropriate shafts allow a plurality of compression stages to be driven jointly, so that only one drive unit has to be provided. The use of a plurality of shafts is advantageous if particular compression stages are to be controlled separately, particularly in the previously mentioned cases.

However, within the context of the present invention, it is also possible for a plurality of the compression stages to be respectively driven by a plurality of common shafts, thereby simplifying a corresponding control. In such cases, a plurality of common shafts can be mechanically coupled together by a transmission, so that for example a particular transmission ratio, which can also be adjustable by an adjustable transmission, can be achieved.

The present invention also relates to a plant which is configured for compressing a gas in stages and to a compressor arrangement which comprises a plurality of compression stages which are connected together sequentially by a main line and in which the gas, guided through the main line, can be respectively compressed from a suction-side pressure level to a pressure-side pressure level and can be heated by this compression from a suction-side temperature level to a pressure-side temperature level, means being provided which are configured to at least temporarily remove a feedback amount of the gas, guided through the main line, from the main line downstream of one of the compression stages, to feed it to an expansion process and to feed it back into the main line upstream of the same compression stage.

According to the invention, the plant is configured to be operated such that the pressure-side pressure level of the compression stage downstream of which the feedback amount is removed from the main line is a supercritical pressure level and the feedback amount is expanded to a subcritical pressure level. Means are provided which are configured to feed the feedback amount to the expansion process at the pressure-side temperature level of the compression stage downstream of which the feedback amount is removed from the main line. Furthermore, means are provided which are configured to cool the feedback amount only after it has been expanded and before and/or after it is fed back into the main line.

A plant of this type benefits from the previously explained features and advantages. It is advantageously configured to implement a method which has been previously described. Therefore, reference is explicitly made to the corresponding features and advantages.

In the following, the invention will be described in more detail with reference to the accompanying drawings, which show embodiments of the invention compared to embodiments which are not according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a multi-stage compressor arrangement according to an embodiment which is not according to the invention.

FIG. 2 shows a multi-stage compressor arrangement according to an embodiment which is not according to the invention.

FIG. 3 shows a multi-stage compressor arrangement according to a particularly preferred embodiment of the invention.

FIG. 4 shows a multi-stage compressor arrangement according to a particularly preferred embodiment of the invention.

FIG. 5 shows a multi-stage compressor arrangement according to a particularly preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In the following figures, mutually corresponding elements have been provided with identical reference signs. For the sake of clarity, they are not described in every figure, unless corresponding elements perform a different function and/or are configured in a different manner.

FIG. 1 shows a multi-stage compressor arrangement according to an embodiment which is not according to the invention and is designated overall by reference sign 500. The compressor arrangement 500 is configured to provide ethylene at a pressure level of approximately 40 bar, i.e. at a subcritical pressure level. As mentioned, the invention is also suitable for compressing other gases such as methane and carbon dioxide. The compressor arrangement 500 comprises a plurality of compression stages which are designated here by Roman numerals I to IV. The compression stages I to IV are connected together by a main line 1. The compression stages I to IV are arranged on a common shaft 8 in the compressor arrangement 500. Ethylene is fed to compression stages I, II and III from ethylene refrigerant circuits at different pressure and temperature levels via corresponding lines 2 to 4. Line 2 conveys ethylene out of a low-pressure refrigerant circuit at approximately 1.05 bar, line 3 conveys ethylene out of a medium-pressure refrigerant circuit at approximately 3 bar and line 4 conveys ethylene out of a high-pressure refrigerant circuit at approximately 8.1 bar.

In the first compression stage I, the ethylene is compressed from the mentioned 1.05 bar, the suction-side pressure level of the first compression stage I, to a pressure-side pressure level of approximately 3 bar, which is at the same time the suction-side pressure level of the second compression stage II. Compression stage II compresses the ethylene in the main line 1 to a pressure-side pressure level of approximately 8.1 bar, which is at the same time the suction-side pressure level of the third compression stage III. In compression stage III, the ethylene is compressed to a pressure-side pressure level of approximately 22.5 bar, which is at the same time the suction-side pressure level of the fourth compression stage IV. In compression stage IV, the ethylene is compressed to a pressure-side pressure level of approximately 40 bar, at which it can be released as product via a line 5. Via the line 4, further ethylene is fed in, for example from the top of a C2 splitter. Since the compressor arrangement 500 is configured as a combined refrigerant and product compressor, an intermediate extraction line 6 is provided for extracting refrigerant and optionally a return flow to the C2 splitter.

To dissipate the compression heat due to the compression in compression stages II to IV, respective aftercoolers IIa to IVa are provided in which the ethylene is respectively cooled to approximately 40° C. Since on the suction side of the third compression stage III cold ethylene is also fed in from the high-pressure refrigerant circuit, upon entry into the third compression stage III a mixed temperature of approximately 18° C. is produced. The entry temperature of the ethylene out of the low-pressure refrigerant circuit into the first compression stage I is approximately −57° C. and the entry temperature of the ethylene out of the medium-pressure refrigerant circuit into the second compression stage II is approximately 14° C.

Via a plurality of return lines 7, feedback amounts can be respectively removed from the main line 1 downstream of compression stages II to IV and can be fed back into the main line 1 upstream of these compression stages. In this respect, the feedback amounts are expanded via valves which are not denoted separately. In a multi-stage compressor arrangement 500 as shown in FIG. 1, the problem of the initially mentioned liquefying effects typically arises to a lesser extent, because a supercritical pressure level is not reached here.

FIG. 2 shows a compressor arrangement 600 according to a further embodiment which is not according to the invention. The compression stages I to IV and the interconnection thereof has already been described. In the compressor arrangement 600, the compression stages I to IV are arranged on a common shaft 8.

In the compressor arrangement 600 according to FIG. 2, two further compression stages V and VI are provided. These are arranged on a common shaft 9 in the compressor arrangement 600 and further compress the ethylene, released via the line 5 as product in the compressor arrangement 500 according to FIG. 1 to a supercritical pressure level. The ethylene, compressed to approximately 40.2 bar and cooled to a temperature level of approximately 40° C., is fed to the fifth compression stage V in the compressor arrangement 600. Thus, the suction-side pressure level of this compression stage V is approximately 40.2 bar. In compression stage V, the ethylene is compressed to a pressure-side pressure level of approximately 70.4 bar from this suction-side pressure level. In so doing it heats up, and is cooled in an aftercooler Va to approximately 40° C. Thereafter, the ethylene is fed to a compression stage VI in which it is compressed to a pressure-side pressure level of approximately 125.6 bar. After cooling in an aftercooler VIa to approximately 40° C., the ethylene is released as product at a temperature level of approximately 40° C. and at the mentioned pressure level via a line 5.

Also provided downstream of compression stages V and VI are return lines 7, by which feedback amounts can be respectively removed from the main line 1 and can be fed back into the main line upstream of the respective compression stages. However, as mentioned, disadvantageous liquefying effects possibly occur during a compression, particularly in compression stage V, during a feedback and an expansion.

In the compressor arrangement 600 according to FIG. 2, the shafts 8 and 9 can be connected together by a transmission, as also shown in the following FIGS. 3 and 4. Thus, the rotational speed of compression stages I to VI can no longer be controlled independently of the other compression stages. If the suction-side pressure level of compression stage V is now reduced, for example because a smaller amount of ethylene is fed in via line 4, this can only be counteracted by opening the return line 7 downstream of the aftercooler Va. This is not a problem provided that it is ensured by the cooling water temperature in the aftercooler Va that the feedback amount, guided in the return line 7 downstream of the aftercooler Va, is at a sufficiently high temperature, for example approximately 40° C. However, in an extreme case, with colder cooling water, the feedback amount, guided in the return line 7 downstream of the aftercooler Va, can fall to a value of for example 20° C. This temperature is further reduced due to the expansion in the expansion valve. The suction-side temperature level of compression stage V thereby also falls, and thus also the suction-side pressure level. Here again, this can only be counteracted by returning a greater feedback amount which in turn, however, causes the suction-side temperature level of compression stage V to fall further. Ultimately, a very large amount of ethylene is circulated without any benefit. This also affects the downstream compressor stages.

FIG. 3 schematically shows a compressor arrangement according to an embodiment of the invention which is designated overall by reference sign 100. The compressor arrangement 100 is largely the same as the compressor arrangement 600 according to FIG. 2. However, whereas in the compressor arrangement 600 according to FIG. 2 the return line 7 is arranged downstream of the aftercooler Va, a corresponding return line, designated here by reference sign 10 for the purposes of clarity, according to the embodiment of the compressor arrangement 100 according to the invention which is shown in FIG. 3, branches off from the main line 1 upstream of this aftercooler and directly downstream of compression stage 5.

This measure can ensure that a feedback amount which is guided through the return line 10 and is branched off from the main line 1 is expanded in an expansion device 11, for example an expansion valve, from a higher temperature level than in the compressor arrangement 600 according to FIG. 2. In this way, no liquefying effects can occur during expansion in the expansion device 11.

Provided downstream of the expansion device 11 in the return line 10 is a separate cooler 12 which can cool the expanded feedback amount in the return line 10. After cooling, the feedback amount is fed back into the main line 1 out of the return line 10.

The embodiment of the compressor arrangement 100 according to the invention which is shown in FIG. 3 also differs from the compressor arrangement 600 according to FIG. 2 in that the common shaft 8 interconnects compression stages I to IV and the common shaft 9 interconnects compression stages V and VI. The shafts 8 and 9 are connected together by a transmission 13. A common drive 14, for example a steam turbine, can thus drive the shaft 8 and the shaft 9. The speed of the transmission 13 can be configured to be variable or fixed.

The disadvantages in terms of control, described with regard to the compressor arrangement 600 according to FIG. 2, are overcome by the embodiment of the compressor arrangement 100 according to the invention which is shown in FIG. 3. Even when the temperature of the cooling water in the aftercooler Va is reduced, it is ensured that the feedback amount, guided in the return line 7 downstream of the aftercooler Va, is not cooled to the great extent mentioned with regard to the compressor arrangement 600 according to FIG. 2. The maximum cooling is restricted by the heat exchanger 12 because subsequently no further expansion takes place. This can prevent an excessive drop in the suction-side temperature level of compression stage V.

FIG. 4 shows a compressor arrangement according to a further embodiment of the invention which is designated overall by reference sign 200. The compressor arrangement 200 according to FIG. 4 is largely the same as the compressor arrangement 100 according to FIG. 3, although here as well, the return line is configured downstream of compression stage VI, just as the return line downstream of compression stage V. For the sake of clarity, the same reference signs are used and reference is made to the above descriptions. As stated above, in the case of compression stage VI as well, undesirable liquefaction is thus avoided which could occur during abnormal operating states, such as start-up or malfunction.

FIG. 5 shows a compressor arrangement according to a further embodiment of the invention which is designated overall by reference sign 300. Like FIG. 4, here the return line 10 branches off directly downstream of compression stage VI and delivers a feedback amount of ethylene to an expansion process in an expansion valve 11. However, here, the ethylene is fed back into the main line 1 directly downstream of the expansion in the expansion device 17, more specifically not directly upstream of compression stage VI, but upstream of compression stage V. A further aftercooler 15 is provided downstream of the ethylene feed-in point of the return flow from the return line 16.

The shafts 8 and 9 of the compressor arrangement 300 are configured separately from one another, separate drives 14 being respectively provided. 

1. A method for compressing a gas in stages in a compressor arrangement having compression stages (I-VI) which are connected together sequentially by a main line and in which the gas, guided through the main line, is respectively compressed from a suction-side pressure level to a pressure-side pressure level and is heated by this compression from a suction-side temperature level to a pressure-side temperature level, a feedback amount of the gas, guided through the main line, being at least temporarily removed from the main line downstream of one of the compression stages (V), being fed to an expansion process, and being fed back into the main line upstream of the same compression stage (V), characterised in that the pressure-side pressure level of the compression stage (V) downstream of which the feedback amount is removed from the main line is a supercritical pressure level, in that the feedback amount is expanded to a subcritical pressure level, in that the feedback amount is fed to the expansion process at the pressure-side temperature level of the compression stage (V) downstream of which it is removed from the main line, and in that the feedback amount is cooled only after being expanded and before and/or after being fed back into the main line.
 2. The method according to claim 1, wherein a further feedback amount of the gas, guided through the main line, is at least temporarily removed from the main line downstream of a further compression stage (VI), is fed to an expansion process, and is fed back into the main line upstream of the same further compression stage (VI), wherein the pressure-side pressure level of the further compression stage (VI) downstream of which the further feedback amount is removed from the main line is a supercritical pressure level, wherein this further feedback amount is expanded to a supercritical pressure level, and wherein the feedback amount is cooled only after being expanded and before and/or after being fed back into the main line.
 3. The method according to claim 1, wherein an additional feedback amount of the gas, guided through the main line, is at least temporarily removed from the main line downstream of an additional compression stage (III, IV), is fed to an expansion process, and is fed back into the main line upstream of the same additional compression stage (III, IV), wherein the pressure-side pressure level of the additional compression stage (III, IV) downstream of which the further feedback amount is removed from the main line is a subcritical pressure level, wherein this additional feedback amount is expanded to a subcritical pressure level, and wherein the feedback amount is cooled only after being expanded and before and/or after being fed back into the main line.
 4. The method according to claim 1, wherein a further heat exchanger is used in a return line used to return the feedback amount and/or in the main line.
 5. The method according to claim 1, wherein the feedback amount is fed back into the main line upstream of one or more compression stages (I-1V) which are arranged upstream of the compression stage (V) downstream of which the feedback amount is removed from the main line.
 6. The method according to claim 1, wherein the feedback amount is controlled based on an attainable or attained suction-side or pressure-side pressure level of one of the compression stages (I-VI).
 7. The method according to claim 1, wherein the gas is cooled between the compression stages (I-VI) using cooling water which is maintained within a predetermined temperature range.
 8. The method according to claim 1, wherein the gas is ethylene or an ethylene-rich gas, which is provided in particular using a steam cracking method, or wherein the gas is ethane or an ethane-rich gas, or wherein the gas is carbon dioxide or a carbon dioxide-rich gas.
 9. The method according to claim 1, wherein a plurality of the compression stages (1-VI) are driven by one or more common shafts, by which the respective compression stages (I-VI) are mechanically coupled.
 10. The method according to claim 9, wherein a plurality of the compression stages (I-VI) are respectively driven by a plurality of common shafts.
 11. The method according to claim 10, wherein the plurality of common shafts are mechanically coupled by a transmission.
 12. A plant which is configured to compress a gas in stages and which comprises a compressor arrangement with compression stages (I-VI) which are connected together sequentially by a main line and in which the gas, guided through the main line, can be respectively compressed from a suction-side pressure level to a pressure-side pressure level and can be heated by this compression from a suction-side temperature level to a pressure-side temperature level, means being provided which are configured to at least temporarily remove a feedback amount of the gas, guided through the main line, from the main line downstream of one of the compression stages (V), to feed it to an expansion process and to feed it back into the main line upstream of the same compression stage (V), characterised in that the plant is configured to be operated such that the pressure-side pressure level of the compression stage (V) downstream of which the feedback amount is removed from the main line is a supercritical pressure level and in that the feedback amount is expanded to a subcritical pressure level, in that means are provided which are configured to feed the feedback amount to the expansion process at the pressure-side temperature level of the compression stage (V) downstream of which the feedback amount is removed from the main line, and in that means are provided which are configured to cool the feedback amount only after it has been expanded and before and/or after it is fed back into the main line.
 13. The plant according to claim 12 which is configured to implement a method for compressing a gas in stages in a compressor arrangement having compression stages (I-VI) which are connected together sequentially by a main line and in which the gas, guided through the main line, is respectively compressed from a suction-side pressure level to a pressure-side pressure level and is heated by this compression from a suction-side temperature level to a pressure-side temperature level, a feedback amount of the gas, guided through the main line, being at least temporarily removed from the main line downstream of one of the compression stages (V), being fed to an expansion process, and being fed back into the main line upstream of the same compression stage (V), characterised in that the pressure-side pressure level of the compression stage (V) downstream of which the feedback amount is removed from the main line is a supercritical pressure level, in that the feedback amount is expanded to a subcritical pressure level, in that the feedback amount is fed to the expansion process at the pressure-side temperature level of the compression stage (V) downstream of which it is removed from the main line, and in that the feedback amount is cooled only after being expanded and before and/or after being fed back into the main line. 